In a multi-cylinder engine that has a plurality of cylinders in one cylinder block, a plurality of combustion chambers are formed on a cylinder block joint surface of the cylinder head along cylinder banks. To each cylinder, an exhaust port for ejecting exhaust gas inside the combustion chamber is connected. There has been a proposition to collect these exhaust ports inside the cylinder head to form an exhaust collecting portion, and make that exhaust collecting portion integral to the cylinder head. In Japanese Patent Application Publication No. 2010-209749, a first water jacket for cooling the surrounding of the combustion chambers is provided. Also, the exhaust collecting portion is integrally formed inside the cylinder head. An upper side flow passage portion is formed on an engine upper side of this exhaust collecting portion, and a lower side flow passage portion is formed on an engine lower side of this exhaust collecting portion. The coolant flows into these upper side flow passage portion and lower side flow passage portion. A communication flow passage portion for communicating between the upper side flow passage portion and the lower side flow passage portion is arranged on a side of the exhaust collecting portion. The upper side flow passage portion, the lower side flow passage portion and the communication flow passage portion constitute a second water jacket for cooling the exhaust collecting portion, separately from the first water jacket. The cooling performance of the exhaust collecting portion is improved by this second water jacket.
In the cylinder head as described in Japanese Patent Application Publication No. 2010-209749, the communication flow passage portion for communicating between the upper side flow passage portion and the lower side flow passage portion is arranged in a position closer to an outer side surface of the cylinder head than the exhaust ports. For this reason, at a time of the molding, it is necessary to set a core for first water jacket and a core for exhaust ports in a mold, and then set a core for second water jacket in the mold after that. Consequently, time and effort are required for the set up of the mold by setting a plurality of cores in the mold at a time of the molding, so that the yield and the productivity of the cylinder head is lowered. Also, when the upper side flow passage portion, the lower side flow passage portion and the communication flow passage portion are to be formed by setting a plurality of cores in the mold in this way, the core for first water jacket and the core for second water jacket are going to be placed along the arrangement direction of the cylinders. For this reason, the upper side flow passage portion and the lower side flow passage portion that are formed by the core for second water jacket are going to be divided in the arrangement direction of the cylinders. Also, the communication flow passage portion can be formed only on an outer side surface side of the cylinder head on a side of the exhaust ports, rather than on a side of the exhaust collecting portion. In other words, it is difficult to form the upper side flow passage portion, the lower side flow passage portion and the communication flow passage portion in such a way as to substantially surround the entire surrounding of the exhaust collecting portion, so that there is a room for improving the cooling performance of the exhaust collecting portion in particular.